The overall objective of this proposal is to determine the influence of blood flow on the basic components of blood which interact with vascular surfaces in the process of thrombosis. The deposition of platelets and proteinaceous material on vessel segments denuded of their endothelial lining will be investigated in an annular perfusion system under the full range of shear conditions encountered intravascularly. Whole blood will be prepared by various techniques: (1) in citrate anticoagulant, (2) native, without anticoagulant, (3) reconstituted with various cellular and proteinaceous fractions of blood, and (4) re-calcified to permit activation of the coagulation system. This blood will be exposed to various types of vascular segments: (1) rabbit, human and pig vessels stored in buffer, (2) freshly prepared vessels, (3) vessels treated with antibodies to selected elements of subendothelium, and (4) vessels deficient in components such as von Willebrand's factor (VWF). The interaction of the vessel surface with fibrin and platelets will be quantified using a morphometric means of evaluation. The activation of fibrinogen and the release of platelet granular material in the blood which has passed over the vascular segments will be measured by radio-immuno assay and correlated with the morphometric results. Special emphasis will be placed on investigating the role of von Willebrand factor (VWF) with respect to the interaction of platelets with subendothelium and the growth and stability of platelet thrombi on vessel walls utilizing a unique group of patients with various well-defined subtypes of VWD. The influence of blood flow on the ability of plasma VWF to diffuse through blood, bind at the subendothelial surface and associate with vessel wall VWF will be related to its ability to influence platelet-subendothelial interaction. The potential importance of platelet VWF in mediating the development of platelet-platelet aggregation on subendothelium will be assessed by using patients with varying levels of platelet VWF and by studying patients with severe VWD after transfusion of VWF. The size distribution of vessel wall VWF multimers, their ability to multimerize on subendothelium and their role in platelet binding to subendothelium will also be investigated.